The field of the invention is electronic structure intermediates and methods for manufacturing same, especially including rinsing intermediates and procedures employed during such manufacture.
Modern electronic substrates are generally fabricated in a multi-step process. For example, the manufacture of a printed circuit board (PCB) typically includes plating of a dielectric with at least one metal layer, photolithographic processing of the metal layer, applying flux to the processed layer, and soldering of the processed board. The PCB may hereby be a rigid structure as typically found in common electronic devices such as a radio, a TV set, or a telephone, but may also be a flexible structure including polyamide and polyimide films, as frequently utilized in packaging applications in the microelectronics industry. Most of these steps generally require specific chemicals and particular environments, and some materials employed in one step may interfere with chemicals or procedures utilized in another step. Therefore it is common to clean the electronic substrate during many stages in the processing to avoid unwanted carry-over of material from one step to the next. For example, in a typical process of manufacturing a printed circuit board, soldering fluxes are first applied to the circuit board to ensure firm and uniform bonding of the solder. Although useful to enhance binding of the solder, fluxes usually need to be removed with a cleaning agent from the board after application of the solder to prevent decreased board resistance and potential malfunction of the circuit board.
A wide variety of cleaning agents is known in the art to remove unwanted material. Generally, cleaning agents for electronic substrates contain additives such as surfactants, alcohol derivatives, alkanolamines, or ionic- and/or non-ionic detergents, etc., to assist in the solubilizing of unwanted materials or contaminants. Additives typically speed up or considerably improve the cleaning process, however, in most cases care must be taken to remove the additives after the cleaning step, or they will contribute to undesirable build-up of residual cleaning agent, which may interfere with downstream process steps.
It is known to use microfiltered ultrapure water to flush PCBs and other electronic substrates in order to prevent undesirable build-up of ingredients from the rinsing fluid. Ultrapure water is especially advantageous, because it can be removed by various methods without leaving concentrated material on the surface of the electronic substrate. However, it is commonly found that ultrapure water is a poor rinsing agent with relatively limited capacity to remove residues from an electronic substrate, leading to fluctuations in the amount of residues depending on the intensity of the rinsing step. This is especially apparent when the electronic substrate is manually rinsed.
It is also known to rinse electronic substrates with substantially pure tap water supplied from a public utility source. In general, tap water exhibits better rinsing performance than ultrapure water and is therefore a common rinsing agent in the production of many electronic substrates, including PCBs. Moreover, tap water is inexpensive, and environmentally friendly. When rinsing with tap water is performed on electronic substrates with micro-sized structures (i.e. structures of smaller than 1 mm), micro- or ultrafiltration is frequently employed to remove particulate matter that might impair the structure or function of the electronic substrate.
However, the use of tap water has a severe disadvantage. Tap water, although substantially pure, still contains a multitude of dissolved ionic species and organic contaminants. When evaporated or not completely removed, the dissolved matter in the tap water tends to form residues by precipitation or crystallization. This may pose additional problems, since the chemical composition of tap water is frequently not consistent over time and location. Consequently, the use of tap water is accompanied by fluctuations in the amount of residues left on an electronic substrate.
Although various methods of rinsing electronic substrates using substantially pure water are known in the art, all or almost all of them suffer from fluctuations in the amount of residues left on an electronic substrate. Thus, there is still a need to provide methods and compositions to eliminate fluctuations in the amount of residues on an electronic substrate.
The present invention is directed to a method of rinsing an electronic substrate. In one step, it is recognized that fluctuations in the amount of residues on an electronic substrate are eliminated by adding a buffer to a rinsing fluid. In another step, a buffered rinsing fluid is prepared by (a) providing water from a water source; (b) deionizing the water to produce deionized water; (c) adding a buffer to the deionized water at a concentration effective to eliminate fluctuations in the amount of residues on the electronic substrate. In a further step the electronic substrate is rinsed with the buffered rinsing fluid.
In one preferred embodiment the source water is substantially pure tap water, wherein the tap water is supplied from a public utility facility, deionized on site by reverse osmosis, and sodium bicarbonate is added at a concentration of 400-500 ppm as a buffer to obtain a pH of approximately 8.0.
In another preferred embodiment, the electronic substrate is a flexible polyimide PCB, and more preferably an unfinished PCB after a photolithographic developing step.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.